The present invention relates to a wireless packet communication method and wireless packet communication system.
Wireless LANs have increasingly become widespread due to its convenience and gradually expanded their service areas. As a wireless LAN access technique, the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) technique is adopted because wireless LANs cannot detect a collision.
The CSMA/CA technique waits for a predetermined period of time (this period is called a back-off period) before each wireless terminal transmits packet data to another terminal via a single access point (base station), and then performs a carrier sense operation to check whether the channel is unoccupied. If the channel is unoccupied, packet data is transmitted. After packet data transmission, a back-off process begins again. Subsequently, the same operation is performed as described above to transmit packet data with appropriate a timing for avoiding a collision with the packet data of another terminal. If the carrier sense operation finds that the channel is occupied; the back-off process begins again. Then, the carrier sense operation is performed again after the back-off process. This operating cycle is repeated until the channel is unoccupied.
Each of a plurality of wireless LANs has an independent access point (base station) within an independent service area. A plurality of terminals within the service area establish communication via the access point within the area.
The wireless LANs are independent of each other and the channel (carrier) frequencies used by their respective base stations differ from each other. The wireless LANs are separately positioned. Therefore, when another wireless LAN is to be used, communication is once cut off, and then the communication is resumed via an access point (base station) in a service area for the wireless LAN to be used.
Although the wireless LANs have been independent of each other and separately positioned, it is preferred in recent years that service areas Z61 and Z62 of new wireless LANs 61 and 62 be established among service areas Z51, Z52, and Z53 or wireless LANs 51, 52, and 53 as shown in FIG. 4 to maintain communication while a wireless terminal is moved over an extensive range (as disclosed, for instance, in Nonpatent Document 1).
[Nonpatent Document 1]    “Nikkei Internet Technology”, Nikkei BP, March 2003, pages 102 through 105
However, wireless LANs 51, 52, 53, 61, and 62 are independent of each other and their associated base stations 51a, 52a, 53a, 61a, 62a use different channel (carrier signal) frequencies as described earlier. Further, the CSMA/CA technique is used as the wireless LAN access technique. Therefore, when, for instance, a wireless terminal moves from service area Z51 of wireless LAN 51 to service area Z61 of wireless LAN 61, the wireless terminal arrives in service area Z61 after passing through area Zx where service areas Z51 and Z62 overlap. In this instance, the channel frequency changes, thereby causing a problem of how to perform a carrier sense operation. When the wireless terminal is in area Z61 where the service areas overlap, a plurality of access points (two base stations 51a, 61a, in the present example) exist. Therefore, a problem arises as to how to perform a carrier sense operation and maintain communication.
The present invention has been made to solve the above problems and provides a wireless packet communication method and wireless packet communication system for sensing a carrier signal transmitted from a plurality of base stations in advance and allowing a wireless terminal to move between wireless LANs while maintaining packet communication.